Gas station without pumps

2018 August 14

3D-printed small snifter

Filed under: Uncategorized — gasstationwithoutpumps @ 11:51
Tags: , , , , , ,

To practice using a different feature of OpenSCAD, I tried making a wine glass by rotating a 2D outline around the Z-axis.  OpenSCAD has much more limited 2D capabilities than SVG, allowing only polygons, not smooth curves, so it took me a while to come up with a way to get a decent shape with just a few vertices.

The tiny wine glass looks ok, but the single-layer walls of the bowl leak, and the stem is too springy.

My first design was based on a tall, thin wine glass, but the glass was too tall for the print volume of the Monoprice Delta Mini printer, so I tried printing it scaled down by a factor of 2. The printer had a lot of trouble with this tiny design. Perhaps the biggest problem is that the thin stem is too flexible, so the printer head dragged the bowl around as it was printing. The stick-slip action resulted in a few small holes in the single-layer bowl.

I then tried adjusting the design so that the default settings would still produce the wine glass, but different settings would match a mini-snifter that does fit in the print volume. This mini-snifter has a bowl volume of about 135ml (4.5oz), and the snifter weighs 33g. I printed it at 0.15 mm/layer, with a wall count of 5 in Cura, 6 top and bottom layers, and 10% infill.

The roughness of the surface is probably due to inconsistent feeding of filament, except where the bowl flares out widely. There the filament is drooping a bit due to insufficient support.

The mini-snifter holds water and is a reasonably close approximation to the glass that I was copying.

Side view of the snifter, showing the “seam” that results from Cura starting each layer by moving to the same starting position on the circle.

Bottom view of the snifter, showing the flat surface created by the glass bed added to my printer.

Inside view of the bowl of the snifter.

Snifter on its side, to show the roughness of the printing where the bowl widens unsupported. The vertices of the polygon that is rotated are also clearly visible here as three lines around the bowl.

I’ve included the OpenSCAD source code below, but it can be more easily downloaded from
https://www.thingiverse.com/thing:3049512

// OpenSCAD module for making goblets
// Kevin Karplus
// 2018 Aug 14
// Creative Commons - Attribution - Non-Commercial - Share Alike license.</pre>

function interp(x, y0, y1) = (1-x)*y0 + x*y1;

// Goblet using rotation about Z-axis
// Defaults are for a tall, narrow wine glass: too tall for the
// Monoprice Delta Mini 3D printer
// h is height
// foot_r is radius of foot
// foot_h is height of foot where it joins stem
// stem_r is radius of stem
// stem_h is height of stem where it joins bowl
// bowl_bottom_r is radius of bowl at stem_h
// bowl_mid_r is radius of bowl at h-bowl_mid_depth
// bowl_top_r is radius of bowl at top
// thickness is thickness of sides of bowl.
module goblet(h= 214, foot_r=36, foot_h=12, stem_r=4, stem_h=110,
bowl_bottom_r=12, bowl_mid_r=28, bowl_mid_depth=42,
bowl_top_r=25, thickness=1.5)
{
bowl_depth=h-stem_h;
sphere_width = interp(0.75, bowl_mid_r,bowl_bottom_r) -thickness;
echo( "sphere_width=", sphere_width);

delta_r =interp(0.5,bowl_mid_r,bowl_bottom_r)-bowl_bottom_r;
delta_d =bowl_depth-interp(0.7,bowl_mid_depth,bowl_depth);
echo("delta_d=", delta_d, "delta_r=",delta_r);
alpha = atan( delta_d/delta_r);
echo ("alpha=", alpha);
sphere_radius= sphere_width/sin(alpha);
sphere_height = stem_h+sphere_radius;
echo( "sphere_radius=", sphere_radius);

difference()
{
rotate_extrude($fa=2)
{ polygon(
points=[
// foot
[0,0], [foot_r,0], [foot_r,thickness],
[foot_r-4, thickness],
[stem_r*1.5, foot_h- 0.5*stem_r],

// stem
[stem_r,foot_h],
[stem_r, stem_h*0.75],
[interp(0.5,stem_r,bowl_bottom_r), 0.9*stem_h],

//bowl
[bowl_bottom_r,stem_h],
[interp(0.5,bowl_mid_r,bowl_bottom_r),
h-interp(0.7,bowl_mid_depth,bowl_depth)],
[interp(0.2,bowl_mid_r,bowl_bottom_r),
h-interp(0.5,bowl_mid_depth,bowl_depth)],
[bowl_mid_r,
h-interp(0.2,bowl_mid_depth,bowl_depth)],
[bowl_mid_r,
h-interp(-0.07,bowl_mid_depth,bowl_depth)],
[bowl_top_r, h],
[bowl_top_r-thickness, h],
[bowl_mid_r-thickness,
h-interp(-0.07,bowl_mid_depth,bowl_depth)],
[bowl_mid_r-thickness,
h-interp(0.2,bowl_mid_depth,bowl_depth)],
[interp(0.2,bowl_mid_r,bowl_bottom_r) -thickness,
h-interp(0.5,bowl_mid_depth,bowl_depth)],
[interp(0.5,bowl_mid_r,bowl_bottom_r)-thickness,
h-interp(0.7,bowl_mid_depth,bowl_depth)],
[interp(0.75,bowl_mid_r,bowl_bottom_r)-thickness,
h- interp(0.85,bowl_mid_depth,bowl_depth)],
[0,stem_h]
]);
};
translate([0,0,sphere_height])
color("red")
intersection()
{ sphere(r=sphere_radius, $fs=0.1);
translate([0,0,-2*sphere_radius+delta_d])
cube([10*sphere_radius, 10*sphere_radius, 2*sphere_radius],
center=true);
}
}

}

// The default example is for a small snifter, with an interior volume of about 135ml.
// Printed in PLA with 0.15mm layers, wall line count 5, top and bottom layer count 6, and 10% infill, it weighs 33g.
goblet(h=95, foot_r=29, foot_h=10, stem_r=4.5, stem_h=30,
bowl_bottom_r=18, bowl_mid_r=33, bowl_mid_depth=40,
bowl_top_r=23, thickness=1.8);
<pre>
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2018 August 3

Tool for sweeping roof valleys

Filed under: Uncategorized — gasstationwithoutpumps @ 21:42
Tags: , , , , ,

As I mentioned in Fixing Monoprice Delta Mini end stops,

I’ve also been working on creating broomstick threading on the printer, both to test the ability of the printer to do screw threads and to make a socket and bracket for attaching a broom head to a telescoping pole for cleaning the valleys of my tile roof.  I’ll post on that in a separate post, with pictures, once I get the whole thing working.

I designed the threads using OpenSCAD, which has no built-in support for creating helical structures like threaded rods and sockets. I found an open-source thread module on the web, but I found its parameterization awkward, so I ended up writing my own.

Here are my practice pieces for designing threaded rods and sockets with OpenSCAD, and printing them on the Monoprice Delta Mini.

All the test pieces were printed with the threads spiraling around the vertical Z-axis, to minimize the amount of unsupported filament. The undersides of the threads are a bit rough, but the roughness can be minimized by making the threads have a nearly triangular profile, so that only a little bit of filament is unsupported in each layer.

The first threaded rod, at the top left, screws easily into a broom head, and has fairly smooth surfaces, but was a bit too loose (diameters a little too small) and only a rather thin part of the thread was taking most of the contact force. The second one was sturdier, but still too loose.  The third one was too fat and would not screw into a broom head, but the fourth one was a good fit, screwing in easily, but not being wobbly.

The first two sockets worked with the first two printed screws, but not with the steel threads on my broom sticks—there was not enough clearance.  The third one fit fairly well and had smooth threads, but the threads were rather thin, and I felt they would not be sturdy enough. The two sockets with plates were beefier, and were experiments in seeing whether I could print horizontal screw holes.  I printed holes for #8 and #9 flat-head screws (countersunk), and the bridging did not cause problems with loose filaments.

I did have a little problem with the flat plates not printing properly, which I tracked down to the glass plate on the bed rotating a little in the clips (pretty much as Michael Johnson had warned me).  I fixed the problem by adding some dabs of hot-melt glue to hold the clips to the plate.  I have renew the hot-melt glue every few prints, since it does not bond the glass and the PLA permanently, but I find that an acceptable tradeoff for the ease of removing prints from glass rather than the bed that the printer comes with.

Here is the final socket attached to the broom head with 1″ #8 wood screws.

Of course, one socket at the end of the broom head would not be very secure—the length of the broom head makes a pretty big lever arm for twisting, and PLA is not that strong a plastic. So I designed a support for the broomstick at the other end of the broom head.  I could, perhaps, have used a commercial conduit strap, but the screw holes in them are a bit far apart for screwing to the broom head, so I made one that fits the broom handle I’m using precisely.

Here are the practice pieces I made, to make sure that the broom handle would fit and to check that I could make long screw holes. Note the notch on the right-hand example.

The first test piece held the broomstick well, but did not allow the broomstick to be inserted and withdrawn easily—there are a pair of screws on the brookstick that hold on the threaded end, and the screwheads stick out a little. The notch in the second test piece was carefully designed to just allow the screwheads through, but I added 1mm more clearance for the final piece.  The test pieces were printed with thin shells and 10% or 20% infill, and so were very light, but I printed the final pieces with thick shells and high infill (50%?), so that they ended up fairly solid.

The screw head on one side fits through the notch, and on the other side is in the diagonal space left by extending the circular hole to form a U. The block is held in place with 2″ #9 deck screws.

Although the deck screws were supposed to drill their own holes in the wood, I found that I had to drill pilot holes for them—the first one I inserted split the wooden broom head, which I had to glue back together. I also made a mistake in printing the U-shaped piece with the screw holes horizontal, as torquing down the last screw started to split the plastic along the laminations. If I were to reprint it, I would print it “legs-up” so that the deck screws compress the laminations together.

Here is the finished roof-valley sweeper, mounted on the telescoping pole.

The broom head is still usable as an ordinary broom, as the extra hardware does not interfere with inserting a broomstick in the normal way.

I used the roof-valley sweeper today to clean one of the roof valleys on my tile roof (the one reachable from the porch roof). It worked well to remove a year’s accumulation of leaves and twigs, though I’ll probably have to use it again this fall, after the trees have shed their leaves. I still have the other roof valleys to clean, which needs to be done with the telescoping pole from a ladder.

I don’t expect the PLA pieces to last forever (this is not UV-resistant material), but I store the broom head in a dark garage, so I expect to get a few year’s use out of them.

I’ve included the OpenSCAD source code below, but it can be more easily downloaded from
https://www.thingiverse.com/thing:3033594

The threaded_rod module:

// All dimensions in mm

// build a polygon for the cross-section of the thread 
// perpendicular to the thread.  
// The outer edge of the thread is on x=0 and all coordinate <=0
module thread_cross_section(height, pitch, base_fraction, top_fraction, pitch_angle)
{
    scale = cos(pitch_angle);
    scaled_top  = top_fraction*pitch*scale;
    scaled_base = base_fraction*pitch*scale;
    polygon(points= [ [-height,0], 
        [0, -(scaled_base-scaled_top)/2],
        [0, -(scaled_base+scaled_top)/2],
        [-height,-scaled_base]]);    
}

// one segment of the thread for the threaded rod, but oriented along the z-axis
module thread_segment(height, pitch, base_fraction, top_fraction, pitch_angle, segments_per_turn)
{
    length = pitch/(sin(pitch_angle) * segments_per_turn);
    // echo("pitch=",pitch, "pitch_angle=",pitch_angle, segments_per_turn,"segments, length=",length);
    linear_extrude(height=length, slices=1)
    {   thread_cross_section(height=height, pitch=pitch,
            top_fraction=top_fraction,
            base_fraction=base_fraction,
            pitch_angle=pitch_angle);
    }
}


// make a long cone for intersecting with screw to trim z=0 end
module chamfer_cone(inner_diam, outer_diam, chamfer_length, total_length)
{
    cylinder(d1=inner_diam, 
             d2=inner_diam+ (outer_diam-inner_diam)*total_length/chamfer_length,
            h=total_length, $fn=60);
}

// Build a threaded rod with trapezoidal threading
//       ____________                  _____________
//  ____/            \_______________/
// Threads start at z=0 on the x axis.
module threads(
        //Screw thread dimensions:
        //Length of Screw
        screw_length = 25.0,

        // Diameter at base of threads
        inner_diam = 13.5,
        // diameter at top of threads
        outer_diam = 18,

        //Thread Pitch
        pitch = 5.0,
        // Thread fraction (thickness of thread at base/pitch)
        base_fraction=1,
        // Thread fraction (thickness of thread at top/pitch)
        top_fraction=0,

        // how much of turn is made per segment of result
        degrees_per_segment=6,

        // chamfer at start (Z=0)  (defaults to pitch/2, use 0 to turn off)
        chamfer_start=-1,
        // chamfer at end (Z=screw_length)
        chamfer_end=0
        )
{
    pi= 3.1415926535897932346;
    pitch_angle = atan(pitch/ (pi *outer_diam));
    
    augmented_length = screw_length+pitch;
    turns = augmented_length/pitch;
    segments_per_turn = 360/ degrees_per_segment;
    segments = ceil(turns*segments_per_turn);
    
    // set the lengths for the chamfers if defaults needed
    local_chamfer_start = chamfer_start<0? pitch/2: chamfer_start;
    local_chamfer_end = chamfer_end<0? 0: chamfer_end; intersection() { if (local_chamfer_start>0)  
        {   chamfer_cone(inner_diam=inner_diam, 
                outer_diam=outer_diam, 
                chamfer_length=local_chamfer_start,
                total_length=augmented_length);
        }
        else
        {    cylinder(d=2*outer_diam, h=augmented_length);
        }
        translate([0,0,screw_length])   
            rotate([180,0,0]) 
                if (local_chamfer_end>0) 
                {     chamfer_cone(inner_diam=inner_diam, 
                    outer_diam=outer_diam, 
                    chamfer_length=local_chamfer_end,
                    total_length=augmented_length);
                }
                else
                {   cylinder(d=2*outer_diam, h=augmented_length); 
                }
        union()
        {
            color("red") cylinder(d=inner_diam+0.001, h=screw_length, $fn=segments_per_turn);
            for (i = [0:segments])
            {   angle = i*degrees_per_segment;
                translate([outer_diam*cos(angle)/2,
                          outer_diam*sin(angle)/2,
                          i*pitch*degrees_per_segment/360])
                    rotate([0,0,angle])
                        rotate([90-pitch_angle,0,0])
                thread_segment(height=(outer_diam-inner_diam)/2, 
                    pitch=pitch, 
                    base_fraction=base_fraction,
                    top_fraction=top_fraction, 
                    pitch_angle=pitch_angle, 
                    segments_per_turn=segments_per_turn);
            } 
        }
    }
}

// example
// rotate([-180,0,0]) union()
// {
//    color("green") translate([0,0,25-0.001])
//            cylinder(d=23, h=5,$fn=60);
//    threads(top_fraction=0.3, base_fraction=0.6, 
//        screw_length=25,
//        inner_diam=15, outer_diam=18);
// }

The broomstick socket:

use <threaded_rod.scad>

// create a screwhole for metric screw of given length,
// with head extending in -z direction and screw in +z direction
// Loose=0.10 for loose fit, 0.05 for tight fit, about -0.15 for threaded hole
module screw_hole(metric_size=3, length=10, loose=0.10)
{   union()
    {   cylinder(d=metric_size*(1+loose), h=length*1.1, $fs=0.3);  // body of screw
    translate([0,0,-metric_size]) cylinder(d=metric_size*2.2, h=metric_size*1.1, $fs=0.3);  // counterbore for head of screw
    }
}

// make countersunk hole for flathead screw
// surface on xy plane at (0,0), screw extends in +z direction.
// length is the length of the hole for the threads (past the base of the head)
// diam is the diameter of the hole for the screw threads
// depth is the depth of the countersink
// top_diam is the diameter at the surface (the xy plane)
module countersunk(length=35, diam=5, depth=3, top_diam=11)
{
    union()
    {    cylinder (d=diam, h=length+depth, $fs=0.3);   // hole for screw
        // countersink (adding 0.001 overshoot to avoid coincident faces)
        translate([0,0,-0.001]) cylinder (d1=top_diam+0.2, d2=diam, h=depth+0.1, $fs=0.3); 
    }
}

// make countersunk hole for 10-24 flathead machine screw
module countersunk_10_24(length=35)
{    countersunk(length=length, diam=5, depth=3, top_diam=11);

} 

// make countersunk hole for 8-32 flathead wood screw 
module countersunk_8_32(length=22, diam=4.3, top_diam=7.6, depth=3.2)
{
    countersunk(length=length, diam=3.2, top_diam=7.6, depth=3.2);
}


// example using broom handle thread
module broom_handle()
 {
    rotate([-180,0,0]) union()
    {
        color("green") translate([0,0,15-0.001]) cylinder(d=23, h=5,$fn=60);
        threads(top_fraction=0.35, base_fraction=0.7, 
            screw_length=15,
            inner_diam=15.5, outer_diam=18.5);
    }
}
// makes a threaded rod that matches the one in broom_handle,
// but bloats diameters and threads.
// By subtracting this from an object, we should be able to
// create an acceptable socket for the broom handle.
module bloated_broom_thread(bloat=0.5, depth=20,
	inner_diam=15.5,
	outer_diam=18.5,
	pitch=5,
	top_fraction=0.35,
	base_fraction=0.7)
{
    threads(top_fraction=min(0.95,top_fraction+0.5*bloat/pitch), 
        base_fraction=min(1,base_fraction+0.5*bloat/pitch), 
        screw_length=depth,
        inner_diam=inner_diam+bloat, outer_diam=outer_diam+bloat,
        pitch=pitch, chamfer_start=0);
}

// Make a cylinder with a plate whose outer edge is tangential to the cylinder
// cylinder has axis along z-axis.
// Plate is parallel to xz-plane on positive y side.
// The cylindrical boss has a fillet that goes from the diameter
//   an extra "fillet" mm wider on each side where it touches the plate.
module plate_with_boss(width=55, length=55, boss_diam=30, thickness=5, fillet=2.5)
{
   boss_radius = boss_diam/2;
   fillet_height=boss_radius-thickness;
   union()
   {  
      color("green")cylinder(d=boss_diam, h=length, $fn=90);
      color("red") translate([-width/2,boss_radius-thickness+0.001,0.001]) cube([width,thickness,length]);
      color("blue") translate([-boss_radius-0.001,0,-0.001]) cube([boss_diam+0.002, boss_radius-0.001, length]);
       linear_extrude(height=length)
       {    polygon(points=[
                [boss_radius-0.1, fillet_height+0.1],
                [boss_radius, fillet_height-fillet], 
                [boss_radius+0.25*fillet, fillet_height-0.5*fillet],
                [boss_radius+0.5*fillet, fillet_height-0.25*fillet],
                [boss_radius+fillet, fillet_height]
           ]);
       }
       linear_extrude(height=length)
       {    polygon(points=[
                [-boss_radius+0.1, fillet_height+0.1],
                [-boss_radius, fillet_height-fillet], 
                [-boss_radius-0.25*fillet, fillet_height-0.5*fillet],
                [-boss_radius-0.5*fillet, fillet_height-0.25*fillet],
                [-boss_radius-fillet, fillet_height]
           ]);
       }
   }  
}

boss_diam=28;
depth=30;   // length of threads in socket
width = 60; // width of plate
length = depth+4; // length of plate
thickness = 7;

screw_depth=max(0,thickness-3.5);  // leave thin plate closing hole

screw_x = boss_diam/2+7;
screw_y = boss_diam/2-thickness-0.001;
screw_z = 8;
screw_holes=true;
rotate([180,0,0])
    difference()
    {   plate_with_boss(boss_diam=boss_diam, length=length, width=width, thickness=thickness);
        translate([0,0,-0.01]) bloated_broom_thread(depth=depth);
        if (screw_holes)
        {    
            translate([screw_x, screw_y, screw_z])
                rotate([-90,0,0]) 
                    countersunk_8_32(length=screw_depth);
            translate([-screw_x, screw_y, screw_z])
                rotate([-90,0,0]) 
                    countersunk_8_32(length=screw_depth);
            translate([screw_x, screw_y, length-screw_z])
                rotate([-90,0,0]) 
                    countersunk_8_32(length=screw_depth);
            translate([-screw_x, screw_y, length-screw_z])
                rotate([-90,0,0]) 
                    countersunk_8_32(length=screw_depth);
        }
    }

The guide for the broomstick:

// create a screwhole for metric screw of given length,
// with head extending in -z direction and screw in +z direction
// Loose=0.10 for loose fit, 0.05 for tight fit, about -0.15 for threaded hole
module screw_hole(metric_size=3, length=10, loose=0.10)
{   union()
    {   cylinder(d=metric_size*(1+loose), h=length*1.1, $fs=0.3);  // body of screw
    translate([0,0,-metric_size]) cylinder(d=metric_size*2.2, h=metric_size*1.1, $fs=0.3);  // counterbore for head of screw
    }
}

// make countersunk hole for flathead screw
// surface on xy plane at (0,0), screw extends in +z direction.
// length is the length of the hole for the threads (past the base of the head)
// diam is the diameter of the hole for the screw threads
// depth is the depth of the countersink
// top_diam is the diameter at the surface (the xy plane)
module countersunk(length=35, diam=5, depth=3, top_diam=11)
{
    union()
    {    cylinder (d=diam, h=length+depth, $fs=0.3);   // hole for screw
        // countersink (adding 0.001 overshoot to avoid coincident faces)
        translate([0,0,-0.001]) cylinder (d1=top_diam+0.2, d2=diam, h=depth+0.1, $fs=0.3); 
    }
}

// make countersunk hole for 10-24 flathead machine screw
module countersunk_10_24(length, diam=5, depth=3, top_diam=11)
{     countersunk(length=length, diam=diam, top_diam=top_diam, depth=depth);
} 

// make countersunk hole for #9 flathead wood screw
module countersunk_9(length, diam=4.4, top_diam=8.3, depth=4)
{     countersunk(length=length, diam=diam, top_diam=top_diam, depth=depth);
} 

// make countersunk hole for 8-32 flathead wood screw 
module countersunk_8_32(length=22, diam=4.3, top_diam=7.6, depth=3.2)
{
    countersunk(length=length, diam=diam, top_diam=top_diam, depth=depth);
}

module rounded_posy_cube(x,y,z,radius=1)
{
    difference()
    {    cube([x,y,z]);
         translate([0,y,0]) 
            cube([2*radius,2*radius, 2*z+1], center=true);
         translate([x,y,0]) 
            cube([2*radius,2*radius, 2*z+1], center=true);
    }
    translate([radius, y-radius, 0]) cylinder(r=radius, h=z, $fs=0.1);
    translate([x-radius, y-radius, 0]) cylinder(r=radius, h=z, $fs=0.1);
}

U_diam=26;
inner_depth=28;
thickness=6;

U_radius = U_diam/2;
center_y = inner_depth-U_radius;
outer_height = inner_depth+thickness;

width = 60;
length = 34; // length of plate
thickness = 6;

screw_length=outer_height+2; 

// screw_x = max(U_radius+thickness+2.2, width/2-thickness-2.2);
screw_x=21;   // (moved in slightly to engage wood better)
screw_y = outer_height+0.001;
screw_z = 8;
screw_holes=true;

notch_angle = atan(center_y/U_radius);
notch_depth = 2;
notch_radius= 5;

notch_offset = U_radius+notch_depth-notch_radius;
notch_x = cos(notch_angle)*notch_offset;
notch_y = center_y + sin(notch_angle)*notch_offset;

test=false;   // set to trim the model in Z for fast printing of
    // a size test

rotate([-90,0,0])   // to make this "legs up" when printing
difference()
{   translate([-width/2,0,0]) 
        rounded_posy_cube(width,outer_height, length, radius=3);
    
    // hole for handle
    translate([0,center_y,-0.01])
        cylinder(d=U_diam, h=length+0.02, $fn=100);
    
    //extend hole to base 
    translate([-U_radius,-0.01,-0.02])
        cube([U_diam,center_y+0.02, length+0.04]);
  
    // cut notch to allow screw head through
    translate([notch_x, notch_y, -1])
        cylinder(r=notch_radius, h=length+2, $fs=0.1);
    // cut notch to allow screw head through
    translate([-notch_x, 2*center_y-notch_y, -1])
        cylinder(r=notch_radius, h=length+2, $fs=0.1);

    if (screw_holes)
    {    
        translate([screw_x, screw_y, screw_z])
            rotate([90,0,0]) 
                countersunk_9(length=screw_length);
        translate([-screw_x, screw_y, screw_z])
            rotate([90,0,0]) 
                countersunk_9(length=screw_length);
        translate([screw_x, screw_y, length-screw_z])
            rotate([90,0,0]) 
                countersunk_9(length=screw_length);
        translate([-screw_x, screw_y, length-screw_z])
            rotate([90,0,0]) 
                countersunk_9(length=screw_length);
    }
    
    if (test)
    {   // just include up through the lower screw hole
        translate([-width,-1, screw_z+6])
            cube([2*width, 2*outer_height, length]);
    }
}

2018 July 29

Fixing Monoprice Delta Mini end stops

Filed under: Uncategorized — gasstationwithoutpumps @ 20:17
Tags: , ,

I noticed that the automatic bed leveling on my Monoprice Delta Mini was not really working, and so I researched the problem on the web.  The general idea from several posts was that the end stops were not precisely positioned, and the automatic bed leveling couldn’t deal with that.

There seemed to be two approaches to fixing the problem:

  • Fuss with the end stops mechanically, loosening screws, tweaking position, and retightening them.  Getting that accurate to 0.1mm or better seemed unlikely to me, and one cautionary tale was of someone dropping one of the nuts and not being able to get it back on the screw.  That sounds like the sort of problem I would have, so I was reluctant to try this.
  • Repeatedly measure the bed, tweaking the firmware offset for the end stops.  That sounded more like the sort of thing I could do, and I even found a program on-line that would do it automatically. https://github.com/TechnoSwiss/MPMD-AutoBedLevel-Cal

I downloaded the program and looked at it carefully, to make sure it wouldn’t mess up my printer.  It seemed pretty clean, except for one block that set the steps/mm for the stepper motors.  The step setting would have been way off for my printer (I think it was appropriate for an older version of the software that had a coarser microstep option).  I removed all the code that referred to steps/mm, since  I had already calibrated that with the Make dimensional print (see Monoprice delta 3d printer glass clips).

I also set the initial values of L (the length of the diagonal rod) and R (the radius of the delta tower) to the factory-set values (120.8 and 61.7)reported by my printer (using the M503 command to print settings), so the program would be working with best estimates I had.

I fixed one parenthesis bug (which would not have caused problems in Python 2, but did not work with the print() function in Python 3).

The program works be doing the 4 taps used for bed leveling, and reporting the position of each point, then doing a linear estimate of how to adjust the end-stop offsets and R to make the numbers come out right.  It makes the adjustments and repeats the measurements until the points are all at about the same Z height.

For my printer, the adjustments were X-0.165  Z-2.923 and R=63.138.  The 3mm offset for the Z-tower was about what I expected based on how tilted the first layer of the prints had been.

I switched over to using the newest version of Cura (3.4.1) from the version 3.2.1 that I had been using, since I now understood the printer well enough to be able to add it as a custom filament printer, without needing a bunch of files from the web to define the printer.  I printed the Make dimensional test piece again, sliced by Cura 3.4.1, to check that I hadn’t messed up the setup of Cura.

It printed fine (on the 2nd try—the first try had a typo in the setup code and was trying to print 3mm up in the air), with a flat bottom layer adhering uniformly to the glass plate—indicating that the end-stop corrections were working as hoped. Its dimensions were

design measured x measured y
25mm 25.25mm 24.65mm
20mm 20.05mm 19.75mm
15mm 10.10mm 10.00mm

which indicates that the steps/mm are accurate to about 1%. I could tweak them a little, but I’m not sure that I’d get consistent improvement from doing so.

I’ve also been working on creating broomstick threading on the printer, both to test the ability of the printer to do screw threads and to make a socket and bracket for attaching a broom head to a telescoping pole for cleaning the valleys of my tile roof.  I’ll post on that in a separate post, with pictures, once I get the whole thing working.

2018 July 1

Adding feet to the Monoprice Delta Mini 3D printer

Filed under: Uncategorized — gasstationwithoutpumps @ 09:22
Tags: , , ,

The first functional prints I’ve created from the new Monoprice Delta Mini 3D printer that I bought earlier this week (see Three boxes this morning!) are small plates to attach rubber feet to the bottom of the printer, to raise it off the table and improve airflow.

I looked at what Thingiverse had for accessories for the printer, and several people had designs for feet, but I didn’t really like any of the designs, so I decided to design my own.

Because 3D printing is so slow, I decided not to try to print tall feet, but to use some rubber feet that I bought back in 2012 from Parts Express. (If you follow that link, you’d find that Parts Express no longer has these feet, but has slightly smaller black rubber feet for about 66¢ each.)  This meant that I only had to design and print adapter plates that could be screwed on over the existing  feet, with a central hole for attaching the rubber foot.

The plate attaches with M3 screws. The design calls for replacing the existing M3×8 Phillips head screws with M3×12 socket-head screws, and I decided to recess the sockets into the plate, both for looks and to see whether the printer could bridge over the recess.  (If the printer had not been able to bridge, then I would have made that surface of the adapter plate be flat, and used M3×16 screws instead.)

I spent a fair amount of time measuring the irregular hexagonal end of the uprights of the printer, so that I could match it and the get the screw holes in the right places.  The measurements were not perfectly consistent, so I had to decide which measurements to take as “correct” and which to compute based on the chosen parameters.  I decided that all the angles should be treated as canonical (multiples of 30°), since that seemed like a likely choice for the designers of the printers.  I decided that the two parallel edges and the distance between them would be my other defining parameters, since that allowed easy definition in Cartesian coordinates, which also seemed like a likely choice for the original designers.

I did the design in OpenSCAD, which does not have anywhere near the feature richness of a professional tool like SolidWorks, but which is (for a programmer) much easier to learn to use, and easier to get precise results with.

Unfortunately, OpenSCAD does not produce the pretty renderings that SolidWorks does, so I can’t show you pretty design pictures. I can, however, share the source code for the design, which you can modify to produce different designs, or just compile and print. The code is at the end of this post.

View of the adapter plate from the outside, rendered by Finder’s “Quick Look” on a Mac.

View of the adapter plate from the printer side, showing the countersunk hole in the center, rendered by Finder’s “Quick Look” on the Mac.

My first print was to test whether I had the holes in the right places, and whether the printer was printing things at the specified size.  (I was pretty sure it was not, as I had printed the Make magazine test piece for dimensional accuracy, and had seen that the printer was printing about 3% small.)  The test print was just a 3mm slab, printed with 10% infill and 0.2mm layers for speed.  OpenSCAD made it easy to create this slab, by intersecting the design with a rectangular prism of the appropriate thickness and location.

The two test pieces I printed. Test piece 1 is a little small and has a few holes misplaced.  On test piece 2, you can see a little “stringing” where the unsupported bridging filaments drooped, but the overall integrity of the bridge seemed adequate.

The slab showed me that the printer was indeed printing a little small, and that I had misplaced the hole for the existing printer foot by 1mm and the screw holes further from the outside edge by about 0.5mm.  I moved the holes, figured out how to do scaling in Cura to scale the part by 102.9% when slicing, and did another test print—a 4mm slab that included the end of the recesses for the socket-head screws, so that I could test the overhang capability.  I printed this one with 20% infill.

The final design is a slab 7mm thick, which 5 screw holes: 4 for the M3 socket-head screws, and one for a 10-24 flat-head screw for attaching the rubber foot. The 10-24 hole is countersunk and is at the base of a cylindrical recess deep enough that the head of the screw has clearance from the foot that is already on the base of the printer. Because the existing feet are just stuck on with double-stick tape, it would probably have been easier to remove them rather than make clearance for them.

Outside view of a disassembled leg.

Printer-side view of a disassembled leg.

Because the 10-24 screw will be hard to retighten once the leg is on the printer, I squirted a little low-temperature hot-melt glue onto the nut after tightening it, so that it would not work loose from vibration.

The three printed legs, with the rubber feet attached. If you look closely, you can see a little of the clear hot-melt glue in the right-hand foot, to keep the nut from loosening.

Bottom view of the printer with the legs attached.

Side view of the printer with the legs attached.

Closeup of the printer with the legs attached, showing the greater space now available for airflow.

I printed the legs one at a time, so that in the event of printer failure, I would only have to redo one leg, rather than all three.

To print the legs, you need to open the .scad file with OpenSCAD, render it, and output a .stl file. Then use Cura to slide the model. I chose a layer height of 0.1mm, a wall count of 4 layers (for strength and stiffness), 4 top layers and 3 bottom layers, a concentric top and bottom pattern, 20% infill (a compromise between strength and speed of printing), and no special build-plate adhesion. The top layers are excessive, as that face is buried against the bottom of the printer and does not need to be pretty. The first leg I printed had some sort of printer/communication failure and froze after only one top layer had printed, and it is still a perfectly usable leg.

(Update 2018 July 15: I’ve made the clips available on Thingiverse: https://www.thingiverse.com/thing:3001872)

Here is the code for the adapter plate for extending the legs of the printer:

// Kevin Karplus
// 2018 June 30
// Leg for extending height of Monoprice Delta Printer, to improve airflow under printer.


// create a screwhole for metric screw of given length,
// with head extending in -z direction and screw in +z direction
// Loose=0.10 for loose fit, 0.05 for tight fit, about -0.15 for threaded hole
module screw_hole(metric_size=3, length=10, loose=0.10)
{   union()
    {   cylinder(d=metric_size*(1+loose), h=length*1.1, $fs=0.3);  // body of screw
    translate([0,0,-metric_size]) 
        cylinder(d=metric_size*2.2, h=metric_size*1.1, $fs=0.3);  // counterbore for head of screw
    }
}

// make countersunk hole for 10-24 flathead screw
// surface on xy plane at (0,0), screw extends in +z direction.
// length is the length of the hole for the threads (past the base of the head)
// diam is the diameter of the hole for the screw threads
// depth is the depth of the countersink
module countersunk_10_24(length=35, diam=5, depth=3)
{
    // top_diam is the diameter at the surface (the xy plane)
    // assumes that cone has an apeture of 90°
    top_diam = diam + 2*depth;
    union()
    {    cylinder (d=diam, h=length+depth, $fs=0.3);   // hole for screw
        // countersink (adding 0.1 overshoot to avoid coincident faces)
        translate([0,0,-0.1]) 
cylinder (d1=top_diam+0.2, d2=diam, h=depth+0.1, $fs=0.3); 
    }
}


// polygon that matches the foot of the Monoprice Delta printer,
// with outside edge on the x-axis, with center of edge at origin.
// "base" is the length of the outside edge in mm.
// Long edge of plate parallel to x-axis at y=height, of length long.
//
// The default parameter values were measured from the printer.  
// The angles are taken as canonical, and the parameters were chosen as
// those that could be easily defined on a Catersian coordinate system,
// with the other side lengths calculated from the geometry.
module foot_poly(base=50,long=65, height=32)
{   
    // how wide trapezoid would be without trimmed-off corner
    extra_long= base+ 2* height/tan(60);
    echo("extra_long=",extra_long);
    
    short = (extra_long-long)/2 * cos(30);  // shortest edge
    
    // Width at widest point (right-angle vertex)
    width =long + 2*short*cos(30);
    
    // remaining edge
    side = (width-base)/(2*cos(60));
    
    // How high up is the widest point
    height_at_width= side*sin(60);
    
    echo("edges=", base, side,short, long);
    echo("width=",width, "height=", height);
    echo("height_at_width=",height_at_width);
    polygon(points=[ [base/2,0], 
            [width/2, height_at_width],
            [long/2, height],
            [-long/2, height],
            [-width/2, height_at_width],
            [-base/2,0]]);
}



module foot(screw_length=12)
{
    // For the 4 M3 screws that hold foot plate to delta printer
    hole_length=screw_length-8;  // how much of screw thread is left?
    counterbore= 3;  // depth of counterbore
    thickness = hole_length+counterbore;
    
    old_foot = 3.7;  // height of bore to avoid old foot
    
    difference()
    {
        color ("red")  linear_extrude(height=thickness) foot_poly();

        color("green") translate([22.5,5,counterbore])  
            screw_hole(metric_size=3,length=hole_length);
        color("green") translate([-22.5,5,counterbore])
            screw_hole(metric_size=3,length=hole_length);
        
        color("blue") translate([32.5,25,counterbore])
            screw_hole(metric_size=3,length=hole_length);
        color("blue") translate([-32.5,25,counterbore]) 
            screw_hole(metric_size=3,length=hole_length);
 
        // make hole for old foot
        center=18;
        color("brown") translate([0,center,thickness-old_foot]) 
            cylinder(d=14,h=old_foot +0.1, $fs=0.6);
        
        // make countersunk hole for 10-24 screw for new foot
        color("magenta") translate([0,center,thickness-old_foot])
            rotate([180,0,0]) countersunk_10_24();
        
    }
}


{  foot();  
}

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